1. Field
The disclosed concept pertains to electrical switching apparatus and, more particularly, to circuit breakers, such as, for example, circuit breakers including overvoltage and surge protection.
2. Background Information
Circuit breakers are generally old and well known in the art. Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
When an overvoltage occurs (e.g., without limitation, due to downed power lines; due to loss of a neutral connection at the line side of a residential panelboard or load center), a surge device, which provides protection for the panelboard or load center and to downstream connected electrical loads, can and will pass elevated voltages to those devices.
Metal oxide varistors (MOVs) are electrically connected line-to-neutral in miniature arc fault circuit interrupter (AFCI) and/or ground fault circuit interrupter (GFCI) circuit breakers having, for example, one or two poles.
For example, U.S. Pat. No. 5,293,522 discloses a trip circuit for a single-pole circuit breaker including a line-to-neutral varistor, and a varistor for a trip circuit silicon controlled rectifier (SCR).
U.S. Pat. No. 5,260,676 discloses a trip circuit for a two-pole circuit breaker including two line-to-neutral varistors, and a varistor for a trip circuit SCR.
It is known to employ a MOV in parallel with a trip circuit SCR and in series with a solenoid trip coil in the trip circuit of AFCI and/or GFCI circuit breakers, and a MOV between the load and neutral terminals of such breakers.
U.S. Pat. No. 5,519,368 discloses a ground fault circuit including a coil assembly, a rectifier, a MOV and an SCR in parallel with the MOV.
If the neutral is “lost” (e.g., without limitation, due to an electrical problem; due to a “white” neutral conductor being disconnected from the power bus) in a single-pole, two-pole or three-phase power system, then the line-to-neutral voltage may rise to 208 or 240 VRMS, thereby causing the line-to-neutral MOV(s) in a circuit breaker to fail (i.e., due to an excessive voltage condition of sufficient duration).
U.S. Pat. No. 4,168,514 discloses that as an added measure of fail-safe protection, a varistor is packaged in a thermally coupled relation with a thermal element, typically a bi-metal, of a trip unit. Thus, the heat generated in the varistor by the flow of abnormal leakage current therethrough will also indirectly heat the thermal element, contributing to the thermal tripping of a circuit breaker pursuant to removing the failing varistor from the circuit.
U.S. Pat. No. 5,321,574 discloses that responsive to a threshold of current passing through a MOV, this causes a magnetic trip circuit to trip a circuit breaker before a thermal trip circuit has had an opportunity to trip. The heat transfer from the MOV to the thermal trip circuit of the circuit breaker will not by itself cause thermal tripping. During failure transition of the MOV, the circuit is cleared by the much more rapid magnetic tripping of the circuit breaker, which by itself provides suitable circuit protection.
There is room for improvement in electrical switching apparatus, such as circuit breakers with surge protection.